Measuring based on a capacitive acceleration sensor has proved to have a simple principle and to provide a reliable method in the measuring of acceleration. The capacitive measuring is based on a change in the gap between two surfaces of a pair of electrodes of the sensor. The capacitance between the surfaces, i.e. the capacity for storing electric charge, depends on the area of the surfaces and on the distance between the surfaces. Capacitive measuring can be used already at rather low measuring ranges of acceleration.
The measuring principle of the capacitive sensor essentially affects the precision of measurement of the sensor. One optimal method of measuring for a capacitive acceleration sensor is, in fact, to measure the sensor such, that the charge across the electrodes to be measured remains equal, whereby the electrostatic forces caused by the charge compensate each other and the error of measurement caused by them will be minimized.
Prior art will be described below with exemplifying reference to the appended drawing, in which:
FIG. 1 shows a measuring circuitry of an acceleration sensor arrangement according to prior art.
FIG. 1 shows a measuring circuitry of an acceleration sensor arrangement according to prior art. The measuring circuitry of the acceleration sensor arrangement according to prior art comprises a measured sensor 1, i.e. pairs of electrodes, a charge amplifier 2, an analog integrator 3 and a feedback connection 4.
In the measuring circuitry of an acceleration sensor arrangement according to prior art, the charge amplifier 2 detects the potential difference, which exists between the output signal Vm from the analog integrator 3 and the central electrode of the sensor 1, and transforms the charge, which, due to the potential difference, flows to the amplifier 2 into a voltage. The received voltage is integrated by the analog integrator 3, until a state is reached, where the output signal Vm represents an ideal voltage distribution. Thus, the circuit is in state, where there is a balance of charge (QC1=QC2) between the terminals of the capacitors C1 and C2 of the measured sensor 1.
The measuring circuitry of an acceleration sensor, according to prior art, transforms, out of the value of the capacitances C1 and C2 of the sensor, an analog voltage Vm=Vr*(C1−C2)/(C1+C2), which is proportional to the reference voltage Vr in use, and which has a very linear response in the utilized acceleration sensors.
The problems in the acceleration sensor measuring circuitry according to prior art are the offset errors in the analog integrator, which always, however, affect the functioning of the circuit. In addition, the analog integrator is sensitive to interference, in a multiplexed application, in particular.
There is also a problem in the acceleration sensor measuring circuitry according to prior art concerning the charge amplifier in a need for a large uniform range of operation because of the variation in sensor capacitances, and also, in the multiplexed case, a need for a large bandwidth, which presents a challenge for the design of the charge amplifiers and tends to increase the power consumption in the prestage.